Descriptions of units, abbreviation, terminology, etc. used throughout the present disclosure are summarized in Table 1.
Zeolite is a crystalline hydrated aluminosilicate that can contain other metals in the framework of the zeolite crystal or that can be deposited, exchanged, or impregnated on the zeolite (i.e. on the surface or in the pores). A method for preparing a zeolite comprises (a) preparing an aqueous mixture of silicon oxide and sources of oxides of aluminum; and (b) maintaining said aqueous mixture under crystallization conditions until crystals of zeolite form. In the crystalline structure, there are pores and channels that can be interconnected. The dimensions and configuration of these pores and channels allow access by molecules of certain sizes. Zeolites are used as catalysts for, among other things, isomerization, toluene disproportionation, transalkylation, hydrogenation, alkane oligomerization, and aromatization. Aromatization is a multi-step process that can comprise the steps of dehydrogenation of the hydrocarbon, if the hydrocarbon is saturated, cyclization of the dehydrogenated hydrocarbon, and aromatization of the cyclized hydrocarbon.
One such example of hydrocarbon aromatization is aromatization of naphtha. Naphtha is a mixture mainly of straight-chained, branched, and cyclic aliphatic hydrocarbons, light naphtha having from five to nine carbon atoms per molecule and heavy naphtha having from seven to twelve carbon atoms per molecule. Typically, light naphtha contains naphthenes, such as cyclohexane and methylcyclopentane, and linear and branched paraffins, such as hexane and pentane. Light naphtha typically contains 60 to 99 wt % of paraffins and cycloparaffins. Light naphtha can be characterized as a petroleum distillate having a molecular weight range of 70 to 150 g/mol, a specific gravity range of 0.6 to 0.9 g/cm3, a boiling point range of 50 to 320° F. (10 to 160° C.), and a vapor pressure of 5 to 500 mmHg at room temperature. Light naphtha can be obtained from crude oil, natural gas condensate, or other hydrocarbons streams by a variety of processes, e.g., distillation.
A zeolite can be formed into a catalyst by first forming the zeolite into a shaped zeolite body (e.g., an extrudate) and then further modifying the shaped zeolite (e.g., ion-exchange, metal deposition) to make a catalyst. Alternatively, the zeolite can first be modified (e.g., ion-exchange, metal deposition) and then the modified zeolite can be formed into a shaped catalyst. Forming a shaped zeolite body or shaped zeolite catalyst can be accomplished by binding the zeolite with a refractive oxide or combination of oxides such as Al2O3, SiO2, SiO2—Al2O3, and the like, where, depending on the catalytic reaction for which the catalyst is to be used, certain refractive oxide materials may be preferred over others. For example, a silica binder or a substantially silica containing binder can be used instead of an alumina binder for aromatization reactions. However, such a silica binder may not provide the catalyst with a desired mechanical strength, which can result in deterioration of catalyst integrity for use in commercial reactors.
The mechanical strength of a formed catalyst (e.g., an extruded catalyst) can be measured to ensure its ability to retain physical integrity during its use in a reactor. There are several methods used for measuring catalyst strength. One such method includes measuring flat crush strength by placing a piece between two flat surfaces, applying a compressive load, and measuring the force required to crush the piece. Flat crush is reported as pound force per millimeter (lbf/mm) or pound per millimeter (lb/mm).
Silica-bound formed zeolite catalysts for use in applications, such as the aromatization of light naphtha, often display low or unacceptable mechanical strength for use in certain commercial reactors, for example, less than 2.0 lb/mm for cylinder (⅛-inch (3.2 mm) diameter) shaped extruded catalyst. A zeolite catalyst with improved mechanical strength and a method of making would therefore be desirable. A zeolite with one or both of improved selectivity and conversion for naphtha aromatization would also be desirable.